User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-20
only function if a receiver tray is part of
the system. The overtemperature fault
is controlled by the temperature of the
reject load.
Modulation Loss Fault
If a modulation loss occurs while the
transmitter is in Auto, the system will
go to Standby after a few seconds until
the modulation is returned. The
transmitter will immediately revert to
Operate. An input loss fault applies a
low from the ALC board to the fault
input at J7-5 on the board.
With jumper W1 in place on J10, the
input fault is connected to the red Input
Loss Fault LED DS9, causing it to light,
and to Q16. Q16 is biased off, causing
its drain to go high. The high is wired to
U5B, pin 5, whose output at U5B, pin 4,
goes low. The low is wired to Q18,
which is biased off, and causes the
drain to go high. The high is connected
to U3D, pin 12, which causes the
output at U3D, pin 14, to go high. The
high connects to U5A, pin 1, and, if the
transmitter is in Auto, pin 2 of U5A is
low. With pin 1 high and pin 2 low, the
output of U5A goes low and reverse
biases Q20, shutting it off. The high at
U5C, pins 8 and 9, causes its output at
pin 10 to go low. This low is connected
to U5D, pin 12, and, if the transmitter
is in Auto, pin 13 of U5D is also low.
The lows on pins 12 and 13 cause the
output to go high and forward bias
Q19. The drain of Q19 goes low and
connects the coil in relay K1, causing it
to switch to Standby.
When the input returns, the input loss
fault is removed from the video fault
input at J7-5. With jumper W1 in place
on J10, the base of Q16 goes high. The
red Input Loss Fault LED DS9 on the
front panel will go out. Q16 is biased
on, causing the drain to go low. This
low is wired to U5B, pin 5, and U5B, pin
6, will be low if no ALC fault occurs. The
two lows at the inputs make the output
at U5B, pin 4, go high. The high is
wired to Q18, which is biased on, and
causes the drain to go low. The low is
connected to U3D, pin 12, which causes
its output at U3D, pin 14, to go low.
The low connects to U5A, pin 1, and, if
the transmitter is in Auto, pin 2 of U5A
is also low. With both inputs low, the
output of U5A at pin 3 goes high. The
high forward biases Q20, causing its
drain to go low. The low connects to
the Operate coil on the K1 relay that
switches the transmitter to Operate.
The low at U5C, pins 8 and 9, causes
the output at pin 10 to go high. This
high is connected to U5D, pin 12, and,
if the transmitter is in Auto, pin 13 of
U5D is low. The high on pin 12 causes
the output of U5D to go low and
reverse bias Q19. The drain of Q19
goes high and removes the low from
the Standby coil in relay K1.
Overtemperature Fault
The thermal switch A8-A1 on the
output dummy load A8 connects to J8-
1 on the board. If the temperature of
the thermal switch rises above 170° F,
it closes and applies a low to J8-1. The
low connects to Q3, which is biased off,
and to the red Overtemperature LED
DS6, which is biased on. The drain of
Q3 goes high and connects to pins 11
and 12 of U4B. The high at the input to
U4B causes it to go high and switches
the system to standby. This removes
the Operate enable commands to any
external amplifier trays.
VSWR Cutback Fault
The reflected power sample of the RF
output of the transmitter is connected
to J2, pin 9, of the board. The sample
connects to op-amp U1B, pin 5, which
buffers the signal before it is split. One
of the reflected samples that is split is
connected to J1-5, which is wired to
J10-5 on the rear of the tray, for
remote monitoring. The other reflected
sample that is split is connected to
position 3 on the front panel meter of
the tray. The final remote reflected